MDL-43430 enrol_ldap: Enrolment lost if member of group with parenthesis

As part of fixing MDL-43430, a typo has been detected in the
quoted alphanumeric characters (the '=' and '>' were transposed).
MDL-43430 enrol_ldap: Enrolment lost if member of group with parenthesis

The group names we get from ldap_find_user_groups() can contain invalid LDAP
filter characters. So we need to escape them for LDAP filter usage.
MDL-45416 enrol_ldap: Use null_progress_trace while running phpunit
MDL-43430 enrol_ldap: Enrolment lost if member of group with parenthesis

While testing the fix, it was found that ldap_stripslashes() was broken and
using a deprecated PCRE patter modifier, so it's also been fixed.

Thanks to Damyon Wiese, some units tests have been added to cover
ldap_addslashes() and ldap_stripslashes().
Merge branch 'wip-mdl-45416-m26' of https://github.com/rajeshtaneja/moodle into MOODLE_26_STABLE
MDL-48083 dml: Empty buffers on forced rollback
Merge branch 'wip_MDL-48083_m26_rollback' of git://github.com/skodak/moodle into MOODLE_26_STABLE
Merge branch 'wip_26_mdl-43430' of https://github.com/iarenaza/moodle into MOODLE_26_STABLE
MDL-43430 ldap: whitespace fix
MDL-47930 mod_forum: Detect subscription to subscription disabled forums

Return forums with disabled subscription that you are subscribed to in
forum_get_subscribed forums
MDL-48035 Fix Behat tests regressions after merging en_fix strings
Merge branch 'MDL-48035-26-enfix' of git://github.com/mudrd8mz/moodle into MOODLE_26_STABLE